The present invention relates to a liquid crystal display and a method of manufacturing the same.
Thin Film Transistor (TFT)-Liquid Crystal Display (LCD) comprises an array substrate (TFT substrate), a color filter substrate (CF substrate) and a liquid crystal layer filled between them. The array substrate comprises a first substrate and the color filter substrate comprises a second substrate. The first and second substrate is typically made of glass.
FIG. 1 shows a schematic structure of a conventional TFT-LCD. The TFT substrate comprises a bottom gate TFT device 13 in which a gate electrode is formed on a bottom side. The bottom gate TFT device 13 comprises a gate electrode 6 on a first substrate 11, a gate insulating layer 5a on the gate electrode 6, an active layer 2 of a-Si (amorphous silicon) on the gate insulating layer 5a, and a source electrode (S) 41/drain electrode 42 (D) on the active layer 2. In order to reduce contact resistance between the S/D layer and the active layer 2, an ohmic contact layer 3 of N+a-Si with good conductivity may be formed between the S/D layer and the active layer so as to form a good ohmic contact with the S/D layer. The TFT substrate further comprises a passivation protective film 10 and a pixel electrode 7 on the passivation protective layer 10. The pixel electrode 7 is connected to the S/D layer through via hole 15 formed in the passivation protective layer 19 and is typically formed of transparent conductive materials such as indium tin oxide (ITO).
The color filter substrate comprises a black matrix 1, a color filter layer 9, and a counter electrode 8 in this order on a second substrate 12. The counter electrode 8 is typically formed of transparent conductive materials such as indium tin oxide (ITO). The manufacturing process of the color filter substrate is relatively simple.
A 5mask process is usually used for manufacturing the TFT substrate. The 5mask process comprises: forming a gate electrode pattern with a gate mask; etching an active layer of a-Si layer and a N+ a-Si layer with an active mask to form an active pattern; forming a SD electrode with SD mask; forming via holes by etching with a passivation protective mask; and forming a ITO pixel electrode layer with an ITO mask. Corresponding exposure processes used in the 5mask process are all conventional exposure method.
In the conventional thin film transistor (TFT)—liquid crystal display (LCD) described above, there are three issues to be concerned. Firstly, a-Si of the active layer in TFT device is a photosensitive material and the conducting channel is located in the uppermost layer of the bottom gate TFT device. Therefore, external lights can easily strike on the a-Si serving as the conducting channel during operation of TFT-LCD, resulting in deterioration of off-state property of the TFT device and having an adverse effect on standing property of the TFT-LCD, even causing image flickering and grey level variation in severe conditions. Secondly, since the TFT-LCD generally operates in a normal white mode for TN-type liquid crystal, image contrast would be decreased substantially due to light leakages occurred in non-display regions when displaying a black image. Thirdly, the liquid crystal subjected to a horizontal electric field between signal lines and pixel electrode would suffer a reverse-tilt alignment defect at edges of the pixel, which would decrease the image contrast and sometimes inducing afterimage. Thus it is necessary for the black matrix (BM) on the color filter substrate to overlap (shown as d1, d2 in FIG. 1) substantially an area where the pixel electrode 7 is formed on the TFT substrate, so as to ensure no light leakage. However, such overlap reduces aperture ratio of the TFT-LCD, which is disadvantageous for enhancing display quality of the device.